Unlike disease cases caused by a PAH mutation, patients with BH4 deficiency and HPA mainly have causal autosomal recessive mutations in the genes encoding the BH4 biosynthesis enzymes, including PTS (6-pyruvoyl-tetrahydropterin synthase) and GCH1 (GTP cyclohydrolase I), or the BH4 regeneration enzymes, including PCBD1 (pterin-4a-carbinolamine dehydratase gene) and QDPR (dihydropteridine gene). The gene discussed is PCBD1; the disease is pulmonary arterial hypertension.